Polysaccharide Storage Myopathy (PSSM) is a novel inherited glycogenosis characterized by myoplasmic accumulation of abnormal polysaccharide and clinical myopathy. We have identified a gain of function mutation in the GYS1 gene encoding the skeletal muscle is form of glycogen synthase in horses with PSSM, which results in an amino acid substitution in a highly conserved region of the enzyme. We have also identified a second, unique non-GYS1 form of PSSM in about 20% of cases. Horses are an ideal model for the study of heritable muscle disease due to extended accurate pedigrees from founder stallions, their natural athleticism which makes metabolic disease readily apparent, and the ease of performing muscle diagnostic testing. The overall goal of this project is to use both forms of PSSM to define new mechanisms regulating skeletal muscle glycogen synthase activity and glycogen metabolism in health and disease. Specific Aim 1 of this project will identify the functional basis for the GYS1 mutation through protein expression in E. coli and GS activity measurements, and Specific Aim 2 will determine the genetic basis of the non-GYS1 form of PSSM through whole genome association mapping with SNP markers. Candidate and environment: Dr. Molly McCue has DVM, MS and PhD degrees, and is board certified in Large Animal Internal Medicine. She has secured a tenure track faculty position with a research program focusing on the clinical, epidemiologic, genetic and functional basis of large animal models of neuromuscular disease. Her short term goals are to build the skills needed to define the genetic and functional basis of disease. The training program is designed to allow Dr. McCue to develop these skills while also developing other tools needed to succeed in academic medicine. The University of Minnesota has a strong program in comparative medicine and a highly collaborative environment which will be integral to Dr. McCue's success.